(a) Field of the Invention
This invention relates to a method for controlling the temperature of heat generating elements of a thermal printing head for a thermal transfer type thermal printer, and to a control circuit for controlling the temperature of heat generating elements of a thermal printing head. More particulartly, this invention relates to a method and circuit for controlling the temperature of heat generating elements of a thermal printing head which is capable of eliminating adverse effects from the ambient temperature upon the thermal head and enables the obtaining of a constant density in printing qualities.
(b) Description of the Prior Art
A known thermal printer carries out a thermal printing by selectively heating up one or more of heat generating elements so as to print a desired character or symbol on a thermal printing paper. A typical conventional driving circuit for such a thermal printing head including heat generating elements is shown in FIG. 1 wherein only a main portion of the circuit is illustrated for the simplification of description. In the figure, the main circuit comprises a plurality of heat generating elements H.sub.1 to H.sub.n, a gate circuit 2 including the same number of NAND gates as that of the heat generating elements H.sub.1 to H.sub.n, and a shift register 1. The shift register 1 receives a serial printing data P.sub.1 and converts it into a n-bit parallel printing data. Every digit of the n-bit parallel printing data is respectively supplied to one input terminal of the corresponding NAND gate, and a strobe pulse signal S.sub.1 having a suitable pulse width for heating up the elements is applied to the other input terminal of the corresponding NAND gate. Thus, the heat generating elements H.sub.1 to H.sub.n, which are coupled between a voltage source +V and respective output terminals of the NAND gates, are selectively heated up in accordance with the contents of the n-bit parallel data (binary logical levels 0 and 1) supplied from the shift register 1.
The amount of heat to be produced in the heat generating element is determined by the product of electric power and period during which the heat generating element is activated. The effective temperature of the heat generating element, however, is decided depending on the environmental temperature where the element is exposed. As a result, even if the same electric power and period is employed in the heat generating element, the higher the ambient temperature rises, the darker or deeper the thermal printing quality is made, and contrary to the above, the lower the ambient temperature falls, the lighter or thinner the thermal printing quality is made.
In view of the problem above, it has been proposed to mount a thermal detector, such as a thermistor or the like, at the vicinity of the heat generating elements. In accordance with the output from the thermal detector which correctly follows the change of the ambient temperature, the adjustment of the pulse width of the strobe signal S.sub.1 or of the voltage value of the voltage source +V is carried out so as to control the heat to be generated in the heat generating elements H.sub.1 to H.sub.n. Thus, a constant printing quality is maintained regardless of the ambient temperature change.
Such a conventional method for compensating the ambient temperature change, however, is applicable only to those types of thermal printers where thermal printing is performed directly onto a thermal printing paper. This is because thermal printing papers available in the market need not require a large amount of heat to obtain allowable printing quality and have a wide operative heat range. Apart from the direct thermal printing as above, a thermal transfer printing has been widely adopted in the art wherein thermal printing is carried out indirectly by transferring heat-dissolving ink contained in a transfer film onto a printing paper. In order to obtain a good printing quality by utilizing a transfer film presently available in the market, it has been a common practise to power the heat generating element up to its maximum rating. Otherwise, sufficient heat energy could not have been produced for a good printing quality. The reason is that thermal transfer efficiency is relatively poor when compared with the direct thermal printing, whereby a substantially large amount of heat is required to obtain a good printing quality and only a narrow operative range can be permitted.
In the latter thermal transfer method, it has been found not satisfactory in that the conventional method for compensating the ambient temperature change described above can not be applied to. In other words, the operating temperature of the heat generating element is set nearly at the maximum, that is, approximately the widest possible pulse width or highest possible voltage are commonly used respectively for the strobe signal S.sub.1 or the voltage source +V. In this situation, if the ambient temperature goes low below the anticipated one, then the heat to be produced in the heat generating element must be increased in order to compensate the ambient temperature change and to restore the previous printing quality. But, there is no room for both pulse width and source voltage to accommodate a necessary adjustment. Conversely, if the ambient temperature goes high over the anticipated one, the conventional method can be applied to compensate the ambient temperature change by either narrowing the pulse width or by decreasing the source voltage. However, in practice, it is difficult to effectively dissipate heat from the heat generating element by such a conventional method, particularly when a continuous long term printing is being performed. Thus, the temperature at the thermal printer head including the heat generating elements is unavoidably forced to rise by a gradual accumulation of heat, thereby causing a dark or blackish printing paper.
In a preferred example of the present invention which will be described hereinunder in detail, the method for controlling the temperature of heat generating elements of a thermal transfer type thermal printing head which elements are used for effecting a thermal transfer of heat-dissolving ink onto a printing paper being delivered along a platen surface, comprises the steps of: (a) comparing the temperature of said thermal printing head with first and second predetermined temperatures, said first temperature being higher than said second temperature; (b) when the temperature of said thermal printing head goes lower than said second temperature, lifting up said printing head apart from said platen and subsequently heating up said heat generating element by feeding an electric power thereto; and (c) when the temperature of said thermal printing head goes higher than said first temperature, actuating a blower to force said element to be air-cooled.
The foregoing and other objects, the features and the advantages of the present invention will be pointed out in, or apparent from, the following description of the preferred embodiments, considered together with the accompanying drawings.